1. Field of the Invention
This invention relates to a fusing device.
2. Description of Related Art
A fusing device in an image forming apparatus such as, e.g., a printer, a photocopier, a printing apparatus, or the like, for example, a fusing device of a printer, conventionally has a fusing roller installed inside with a heating source, and a pressure roller driven to rotate in pressurized contact with the fusing roller, and in such a fusing device, a sheet carrying a toner image not yet fused is transmitted to a nipping area formed between the fusing roller and the pressure roller to fuse the toner image onto the sheet at the nipping area.
Where the fusing device of this heating roller type is applied to a multicolor printer, a predetermined thermal amount is required to fuse the toner image, and because the toner images in multiple colors need to be overlaid on the sheet to form the multicolored image, a required thermal amount undesirably increases by that much. Furthermore, where the fusing device of the heating roller type is applied to a high speed printer, since the toner image is fused at a high speed, a required thermal amount per unit time further increases compared with a case where the toner image is fused at a low speed.
The thermal amount required for fusing toner image is generally determined by a power (watt=the thermal amount/time) of the heating source at the nipping area as well as each factor such as a nipping width and duration of pressurized contact. Therefore, to increase the thermal amount given to the toner image, it is required to increase the power of the heating source, to widen the nipping width, or to lengthen the duration of pressurized contact. Furthermore, to fuse the toner image at high speed, it is required to increase the power of the heating source or to widen the nipping width by an amount corresponding to shortening of time.
To increase the power of the heating source, it is required to enhance thermal resistance of constructional components of the fusing device or to reduce power consumption thereof, and when the multicolored toner images are fused at high speed, it becomes important to widen the nipping width.
To fuse the multicolored images at a high speed, a fusing device of a belt type has been provided in which a fusing belt is used to wide the nipping width (ref., e.g., Japanese Patent Application Publication No. JA-H11-282,293).
FIG. 2 is a cross-sectional view showing a conventional fusing device of a belt type, and FIG. 3 is a cross-sectional view of a conventional fusing belt.
In FIG. 2, numeral 11 is a heating roller rendered to rotate in a direction of arrow H; numeral 13 is a fusing belt; numeral 14 is a fusing roller rendered to rotate in a direction of arrow G; and numeral 15 is a pressure roller rendered to rotate in an direction of arrow K The fusing belt 13 is arranged as tensioned between the heating roller 11 and the fusing roller 14, and driven in a direction of arrow J. The pressure roller 15 is urged with a spring 19 and made in pressurized contact with the fusing roller 14 through the fusing belt 13, and a prescribed nipping area n is formed between the fusing belt 13 and the pressure roller 15. A sheet 22 is conveyed in a direction of arrow S, transmitted to the nipping area n, and nipped between the fusing roller 14 and the pressure roller 15 through the fusing belt 13 at the nipping area n. The heating roller 11 is urged with a spring 21 in the direction opposite to the fusing roller 14 to render the fusing belt 13 tensioned.
The heating roller 11 has inside a heater 12 such as, e.g., a halogen lamp and heats the fusing belt 13. To heat the fusing belt 13 effectively, the heating roller 11 is formed of a metal having a high thermal conductivity, such as, e.g., aluminum, iron, or the like.
The fusing belt 13 provides the heat received through the heating roller 11 for the toner image 20 at the nipping area n, thereby fusing the toner image 20 formed on the sheet 22 onto the sheet 22.
As shown in FIG. 3, the fusing belt 13 is composed of an external layer 13a, an intermediate layer 13b, and an internal layer 13c, in which the external layer 13a is made of a resin having a high mold releasing property, such as, e.g., a perfluoroalkylvinylether copolymer resin (hereinafter referred to as a “PFA”), and suppresses adherence of toners to the fusing belt 13. Furthermore, the intermediate layer 13b is made of a resin having an elasticity, such as, e.g., silicone rubber, while the internal layer 13c is made of material having a rigidity as well as a thermal resistance, such as a metal, e.g., nickel, stainless steel, etc., or such as a resin, e.g., polyimide, etc. and is thinned to obtain flexibility, for example, to have a thickness of 30 to 50 microns in a case of a metal or to have a 50 to 100 microns in a case of a resin.
The fusing roller 14 is provided with a core metal 14b made of a metal such as, e.g., aluminum, iron, or the like, and an elastic layer 14a made of silicone rubber in a form of sponge, or the like, formed on a surface of the core metal 14b. The elastic layer 14a is rendered in a sponge form to take air inside, thereby having thermal insulation property to suppress an increase in temperature of the fusing belt 13, as well as enlarging the nipping area n. The elastic layer 14a furthermore pushes uniformly toner layers having different colors and thicknesses respectively, composing the multicolored toner image 20 when fusing the multicolored toner image 20.
The pressure roller 15 is provided with a core metal 15b composed of a tube of a metal, e.g., aluminum, iron, or the like, and an elastic layer 15a made of silicone rubber or the like, formed on a surface of the core metal 15b, and has inside a heater 16 such as, e.g., the halogen lamp. The elastic layer 15a pushes uniformly the fused toner layers having different thicknesses respectively at a time of double side printing.
Numeral 17 is a separation piece disposed as in contact with the fusing belt 13, and the separation piece 17 separates the sheet 22 from the fusing belt 13 in a case where the sheet 22 to be delivered from the nipping area n in a direction of arrow E wrapps around the fusing belt 13.
Operation of the fusing device thus structured is explained next.
The heaters 12, 16 are first rendered to generate heats to heat the heating roller 11 and the pressure roller 15. Next, the fusing roller 14 is rotated in the direction of the arrow G by driving a motor, not shown, so that the fusing belt 13 is driven to move in the direction of the arrow J in accordance with the fusing roller 14; the heating roller 11 is driven to rotate in the direction of the arrow H in accordance with the fusing belt 13; and the pressure roller 15 is driven to rotate in the direction of the arrow K in accordance with the fusing belt 13.
When the temperature of the fusing belt 13 heated with the heating roller 11 and the pressure roller 15 reaches the set fusing temperature, the sheet 22 is conveyed in the direction of the arrow S to fuse the toner image 20 onto the sheet 22 at the nipping area n, and subsequently the sheet 22 is delivered in the direction of the arrow E. It is to be noted that where the sheet 22 wraps around the fusing belt 13, the separation piece 17 separates the sheet 22 from the fusing belt 13.
In the meantime, a pressingly deformed amount of the elastic layer 14a of the fusing roller 14 needs to be increased to widen the nipping width at the nipping area n.
Therefore, the thickness of the elastic layer 14a is made thicker while urging force for rendering the pressure roller 15 in pressurized contact with the fusing roller 12 is made greater with the spring 19.
With the conventional fusing device, however, in a case of a small diameter of the fusing roller 14, the fusing roller 14 bends, thereby causing a difference in the nipping width between a central portion and opposite ends in a longitudinal direction of the fusing roller 14, so that defective fusing comes to easily occur.
It is conceivable to reduce hardness of the elastic layer 14a to widen the nipping width by that much, however, in that case it become impossible to pressure sufficiently the toner image 20 in which the toner layers are overlaid, so that the defective fusing easily occurs as well.
It is an object of this invention to solve the problems on the conventional fusing device as described above, and to provide a fusing roller capable of preventing occurrence of defective fusing.